This invention relates to a method and apparatus for pouring in-ground concrete pilings, and in particular pilings which uniformly contact the surrounding soil over their entire extent and have a substantially constant cross-section throughout their length.
As the cost of steel has increased it has become cost effective to make pilings from concrete by casting them in place in the ground. This is accomplished by driving an elongate mandrel into the ground, filling the resultant hole with fluid grout, and then pulling the mandrel back out of the ground leaving the grout in place to cure. The holding ability of such a piling comes from end-bearing and from the friction which is created by the close contact between its sidewalls and the soil which surrounds it. It has become standard practice to place a foot, having a larger cross-sectional area than the mandrel, at its bottom before it is driven into the ground. The foot then forms an open space around the mandrel which also is filled with grout. In order to fill the hole created by the driving of the mandrel, grout is gravity fed into and/or around the mandrel as it is being driven into the ground. Examples of these devices are Steding, U.S. Pat. No. 3,851,485 and Poma, U.S. Pat. No. 4,018,056.
A problem with this procedure is that mandrels may be driven into the soil quite quickly, particularly when they are driven into soft soil. As a result, the grout often cannot flow into the hole fast enough to keep up with its formation. When this occurs, voids are formed around the resulting piling which reduce the amount of its frictional soil contacting surface. In addition, if soil or objects in the soil fall into these voids, the resultant pile diameter will neck down at the locations of the voids, adversely affecting the strength of the pile; also the mandrel may be more difficult to pull back out of the ground.
Several prior art devices have attempted to overcome this problem by pumping the grout into the mandrel under positive pressure as the mandrel is being driven. Examples of such devices are shown in Hochstrasser, U.S. Pat. No. 3,084,518 and Federer, U.S. Pat. No. 4,618,289. However, because grout is very viscous, it is difficult to maintain a sufficient volumetric flow rate through the pump to keep up with a fast-driving mandrel. Accordingly, pressure filling by means of a pump may be no more effective than gravity filling in soft soil conditions.
The subject invention overcomes the foregoing shortcomings of the prior art systems for forming in-ground concrete piling by employing a hollow mandrel as a grout reservoir having grout transmitting openings at the bottom of the mandrel. The long narrow mandrel is filled with grout prior to driving, thereby providing a high-volume reservoir which also has high static pressure.
The foot which is placed below the mandrel has a tubular side wall which extends upwardly from its bottom and surrounds the grout openings leaving an annular space between the sidewall of the foot and the mandrel into which grout can flow from the mandrel through the openings. An annular collar which fits slidingly around the mandrel provides a seal preventing grout from flowing out of the annular space while the foot is above ground proximate to the collar, but permitting such flow after the foot has been driven into the ground and is remote from the collar.
In use the mandrel is filled with grout before it is driven into the ground. A small amount of this grout flows out of the openings in the bottom of the mandrel and fills the annular space between the mandrel and the foot and the sliding clearance between the mandrel and the collar. However, the seal between the mandrel and the foot, and a sliding seal between the mandrel and the collar, prevent the grout from flowing out of the annular space. In addition, the collar is sufficiently weighted, or otherwise subjected to a sufficient downward force, that the high pressure created by the head of grout in the mandrel will not lift the collar off of the foot and permit the grout to escape.
After the mandrel is filled with grout it is driven into the ground using standard pile driving techniques. As the mandrel is driven into the ground and the foot forms a space between the mandrel and the soil, this space is filled with grout immediately as it is being formed, even at high driving speeds. This is made possible by two factors. First, the grout stored at a great height in the long, prefilled mandrel creates high static pressure at the bottom of the mandrel which is equal to or greater than that which could be produced by most pumps. Second, because the hollow interior of the mandrel provides enough grout storage preferably to pour the entire piling, or at least the majority thereof, there is no limitation on volumetric flow rate as would be imposed by most pumps. Thus, the invention supplies grout to the space between the mandrel and the soil as quickly as this space is created and the resulting piling thus has no voids and contacts the soil over its entire extent.
Once the mandrel is driven to the required depth it is pulled back out of the ground leaving the grout in place where it will cure into a concrete piling.
According to another aspect of the invention, the mandrel is optionally provided with an upwardly-facing reaming surface adjacent its lower end which, as the mandrel is pulled back out of the ground, compresses any soil, which may intrude into the hole formed by the foot, radially outwardly so that the intrusion is removed. This further ensures against any necking down of the resultant pile diameter.
Accordingly, it is a principal object of the present invention to provide an apparatus for forming in-ground concrete pilings with a removable mandrel which results in the formation of a pile which substantially uniformly contacts the surrounding soil and is of substantially uniform diameter from top to bottom without necked-down areas adversely affecting its strength.
It is a further principal object of the present invention to provide an apparatus for forming in-ground concrete pilings with a removable mandrel in which the fluid concrete is first stored and then flows into the space created around the mandrel as that space is being formed.
It is a further object of the present invention to provide such an apparatus wherein the mandrel is filled with fluid grout before it is driven into the ground.
It is a still further object of the subject invention to provide such an apparatus which prevents the grout from flowing out of the mandrel when it is filled prior to driving.
The foregoing and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.